1. Field of the Invention
The present invention relates to a magnetoresistive device that incorporates a magnetoresistive element and a method of manufacturing such a magnetoresistive device, a thin-film magnetic head that incorporates a magnetoresistive device that functions as a device for reading a magnetic signal and a method of manufacturing such a thin-film magnetic head, a head gimbal assembly, and a hard disk drive.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading.
MR elements include: an AMR element that utilizes the anisotropic magnetoresistive effect; a GMR element that utilizes the giant magnetoresistive effect; and a TMR element that utilizes the tunnel magnetoresistive effect.
Read heads that exhibit a high sensitivity and a high output are required. Read heads that meet these requirements are GMR heads incorporating spin-valve GMR elements. Such GMR heads have been mass-produced.
In general, the spin-valve GMR element incorporates: a nonmagnetic layer having two surfaces that face toward opposite directions; a soft magnetic layer located adjacent to one of the surfaces of the nonmagnetic layer; a ferromagnetic layer located adjacent to the other one of the surfaces of the nonmagnetic layer; and an antiferromagnetic layer located adjacent to one of the surfaces of the ferromagnetic layer that is farther from the nonmagnetic layer. The soft magnetic layer is a layer in which the direction of magnetization changes in response to the signal magnetic field and called a free layer. The ferromagnetic layer is a layer in which the direction of magnetization is fixed by the field supplied from the antiferromagnetic layer and called a pinned layer.
Another characteristic required for the read head is a small Barkhausen noise. Barkhausen noise results from transition of a domain wall of a magnetic domain of an MR element. If Barkhausen noise occurs, an abrupt variation in output results, which induces a reduction in signal-to-noise (S/N) ratio and an increase in error rate.
To reduce Barkhausen noise, a bias magnetic field (that may be hereinafter called a longitudinal bias field) is applied to the MR element along the longitudinal direction. To apply the longitudinal bias field to the MR element, bias field applying layers may be provided on both sides of the MR element, for example. Each of the bias field applying layers is made of a hard magnetic layer or a laminate of a ferromagnetic layer and an antiferromagnetic layer, for example.
In the read head in which bias field applying layers are provided on both sides of the MR element, two electrode layers for feeding a current used for magnetic signal detection (that may be hereinafter called a sense current) to the MR element are located to touch the bias field applying layers.
As disclosed in Published Unexamined Japanese Patent Application Heisei 11-31313 (1999), it is known that, when the bias field applying layers are located on both sides of the MR element, regions that may be hereinafter called dead regions are created near ends of the MR element that are adjacent to the bias field applying layers. In these regions the magnetic field produced from the bias field applying layers fixes the direction of magnetization, and sensing of a signal magnetic field is thereby prevented. Such dead regions are created in the free layer of the spin-valve GMR element.
Consequently, if the electrode layers are located so as not to overlap the MR element, a sense current passes through the dead regions. The output of the read head is thereby reduced.
To solve this problem, the electrode layers are located to overlap the MR element, as disclosed in Published Unexamined Japanese Patent Application Heisei 8-45037 (1996), Published Unexamined Japanese Patent Application Heisei 9-282618 (1997), Published Unexamined Japanese Patent Application Heisei 11-31313 (1999), and Published Unexamined Japanese Patent Application 2000-76629, for example.
It is possible to reduce Barkhausen noise while a reduction in output of the read head is prevented, if the read head has a structure that the bias field applying layers are located on both sides of the MR element, and the electrode layers overlap the MR element, as described above. Such a structure is hereinafter called an overlapping electrode layer structure.
Reference is now made to FIG. 23 to FIG. 27 to describe an example of method of manufacturing a read head having the above-described overlapping electrode layer structure. In this example the MR element is a spin-valve GMR element. In the method, as shown in FIG. 23, a base layer 121, an antiferromagnetic layer 122, a pinned layer 123, a nonmagnetic layer 124, a soft magnetic layer (a free layer) 125, and a protection layer 126 are formed in this order through sputtering, for example, and stacked. After the protection layer 126 is formed, part of the top surface thereof is natural-oxidized and an oxide layer 140 is formed.
Next, as shown in FIG. 24, a resist mask 141 is formed on the oxide layer 140 through photolithography. The resist mask 141 is used for patterning the layers from the oxide layer 140 to the pinned layer 123. Next, these layers are selectively etched through ion milling, for example, using the resist mask 141, and thereby patterned.
Next, as shown in FIG. 25, on the antiferromagnetic layer 122, two bias field applying layers 127 are formed on both sides of the layers from the oxide layer 140 to the pinned layers 123 while the resist mask 141 is left unremoved.
Next, as shown in FIG. 26, the resist mask 141 is removed and the oxide layer 140 is completely removed through dry etching. Next, a conductive layer 129 is formed on the bias field applying layers 127 and the protection layer 126. The conductive layer 129 is made of a material of which electrode layers 106 described later are made.
Next, as shown in FIG. 27, a specific width of the conductive layer 129 between the two bias field applying layers 127 is etched through reactive ion etching, for example, to form a trench 130. The conductive layer 129 is divided into two by the trench 130, and the two electrode layers 106 are thus formed. In the region between the two electrode layers 106, after this etching, at least part of the protection layer 126 is natural-oxidized and made to have a high resistance, so that a high resistance layer 131 is formed.
As described above, when the overlapping electrode layer structure is adopted, it is possible to reduce Barkhausen noise while a reduction in output of the read head is prevented.
However, a problem is that it is inevitable that a sense current flows into dead regions created in the free layer even though the overlapping electrode layer structure is adopted to the spin-valve GMR element in which the pinned layer is located closer to the substrate while the free layer is located farther from the substrate.
A technique disclosed in Published Unexamined Japanese Patent Application 2000-285418 is that the overlapping electrode layer structure is adopted and high resistance layers are formed on sidewalls of the MR element so as to prevent a sense current from passing through the dead regions. However, this technique has a problem that the ability to apply a longitudinal bias field to the MR element is reduced.